What is the hotkey for sculpting grease pencils included in v2.77?
I don't know if you've solved this problem as yet, but the method to sculpt Grease Pencil strokes is as follows:


press 'Enable Editing' in the Grease Pencil tools panel on the Toolshelf or choose Edit Strokes interaction mode in the 3D Viewport header;
choose desired brush (on the Toolshelf > Grease Pencil Sculpt Strokes rollout) 
press and hold the E key whilst pressing the LMB or using the pen of a tablet.


Ensure you still have the RMB as your object selection choice. If you've changed the preference for selection to the LMB then you will need to hold the E key and use the RMB to sculpt your Grease Pencil strokes.